1. Field of the Invention
The present invention relates to an image processing apparatus and a method therefor and, more particularly, to an image processing apparatus for outputting gradation images and a method therefor.
2. Description of the Related Art
An image processing apparatus employing an electrophotographic process, for example, a laser beam printer employing laser scanning technology, has attracted attention from the points of view of high image quality, high-speed print, low noise, and so on. In such an apparatus, to obtain still higher image quality, a method is known of managing resolution and intermediate gradation at the same time by one-dot multi-valued recording.
As one-dot multi-valued recording, there is a method of obtaining smooth continuous gradations by controlling (pulse-width modulating) the laser emission time per one dot with the laser emission intensity (laser current value) maintained at a constant value. Since this method is close to binary recording, it is possible to perform relatively stable recording.
In another example, a full-color printer has been put into practical use, which forms gradation images for each toner of yellow, magenta, cyan, and black, and superimposes the images on one another.
FIG. 1 shows the construction of a pulse-width modulation circuit which performs 8-bit pulse-width modulation at an image clock PCLK of 20 MHz (50 ns).
Referring to FIG. 1, reference numeral 501 denotes a D/A converter. Reference numeral 502 denotes a variable resistor which attenuates the output of the D/A converter 501. Reference numeral 503 denotes a variable resistor which generates an offset voltage OFS. Reference numeral 504 denotes an adder. Reference numeral 505 denotes a triangular-wave generating circuit which generates a triangular wave in synchronization with the image clock PCLK. Reference numeral 506 denotes a comparator.
FIG. 2 shows signal waveforms of each section of the pulse-width modulation circuit, showing a state in which the comparator 506 performs a voltage comparison between a triangular wave /TRI which is generated in synchronization with the image clock PCLK and an output DA of the adder 504 which is proportional to image data VDO (0 . . . 7), and negative pulse-width modulation output /PVDO is obtained.
Toner is adhered by a print mechanism (not shown) in the interval in which the negative pulse-width modulation output /PVDO is at a low level Lo, and a print result shown in FIG. 2 is obtained.
However, in this electrophotographic process, the relation between the pulse width of signals and the print density is not linear, and becomes non-linear as shown in FIG. 3. This non-linear curve is called a .gamma. curve. Accordingly, by making the pulse width area min to max in which the print density varies in 256 steps (for example, the area in which the pulse width is from 20% to 80% is made in 256 steps, i.e., 0.6/256=0.23% steps) rather than making 0% to 100% of the pulse width in 256 steps (i.e., 1/256=0.39% steps), the steps of the pulse in the area where the curve is sharp is made as small as possible so as to improve the gradation of the print density.
This setting of the minimum/maximum pulse width is performed before the product is shipped from the factory. The procedure is as follows: initially, image data VDO (0 . . . 7) is set at 00h, and the variable resistor 503 shown in FIG. 1 is adjusted (offset voltage OFS is applied) so that the negative pulse-width modulation output /PVDO becomes, for example, 20% (10 ns), thus setting the minimum pulse width. Next, image data VDO (0 . . . 7) is set at FFh, and the variable resistor 502 shown in FIG. 1 is adjusted (the output of the D/A converter 501 is attenuated) so that the negative pulse-width modulation output /PVDO becomes 80% (40 ns), thus setting the maximum pulse width.
The performance of pulse-width modulation for making the interval of the minimum/maximum pulse width which is set in this way to 256 steps makes the pulse width variable amount per one step which is 195 Ps (50 ns/256) smaller to 117 ps ((40-10) ns/256), and thus the gradation of the print density is improved.
However, the above-described technology has the following problems. As described above, the setting of the minimum/maximum pulse width is performed by a variable resistor. However, the accuracy required for that adjustment is extremely high, for example, (80-20)%/256=0.23% or more. However, since the resolution of a low-price variable resistor is low, it is difficult to suppress the adjustment error within a range which satisfies the required accuracy, and variations shown in FIG. 4 occur. Also, even if adjustments can be made, the pulse width varies due to temperature change or aging. Further, there are about several % of variations in the semiconductor laser emission threshold value current, and even if the adjustment accuracy of the pulse-width modulation circuit is improved with the variations being ignored, this adjustment is meaningless with respect to reducing the error.